Telephone system



.1924. 1,507,116 Sept 2 E. E. HINRICHSEN TELEPHONE SYS'I'II Filed July 14. 19 4 eta-Sheet 2 Sept. 2 1924. 1,507,116

E. E. HINRICHSEN TELEPHONE SYSTEM Filed July 14. 1920 4 Sheets-Shet 5 /nvem0r*:

Edward E. H hn'chsen I I 1,507,116 2 1924 E; a. HINRICHSEN TELEPHONE SYS'I'II Filed Jul) 14. 1920 4 Sheets-Sheet 4- Marginal //7 venfor: [award 5. Mme/756m.

Patented Sept. 2, 1924.

UNITED STATES PATENT OFFICE.

EDWARD E. HINRIGHBEN, OF NEW YORK,

N. Y., ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, O F NEW YORK, N. Y., A CORPORATION OF NEW YORK.

TELEPHONE SYSTEM.

Application filed July 14,

To all whom it may concern:

Be it known that I, EDWARD E. HIN- nIoHsEN, a citizen of the United States, residing at New York, in the county of New York, State of New York, have invented certain new and useful Improvements in Telephone Systems, of which the following is a full, clear, concise, and exact description.

This invention relates to telephone systems and particularly to subscriber-operated switching means for private branch exchange working.

The object of the invention is to an all rela system of small capacity embodying all the conveniences of a modern automatic telephone system suitable for use in a private residence or small business establishment where the number of stations is so small that the regular employment of an operator is unwarranted.

In fulfilling this object certain novel features have been devised, among which are the following:

In order to establish a connection between substations, a link circuit is emplo ed to which the calling line is automatical y connected and which is directively actuated to connect to a particular called line. This link circuit is provided with battery feed coils and condensers in order to inductively unite the two ends of the conversational circuit while keeping them conductively divided for reasons well known and long recognized in this art. However, when a trunking connection is desired, the link circuit functions in a different manner. It is now used only temporarily and merely assists in selecting an idle central oflice trunk and establishing a direct connection from the calling line thereto after which it is freed for further use in establishing other local or trunking connections.

In develo ing the means for performing these functions a circuit feature has been provide devised which involves a novel 0 eration of certain controlling rela s. Inc uded in the link circuit is an impu se-responsive relay and a holding relay. When the link circuit is used in a local connection, it is freed at the end of the conversational eriod through the deenergization of first, t e impulse-responsive relay and second, the holdmg relay, but when the link circuit is used 1920. Serial No. 896,143.

in a trunking connection, it is freed through the deenergization of first the holding-relay and lastly the impulse-responsive relay. Another feature of this invention resides in a novel means for establishing control over and initiating circuit movement in a link circuit, whereb several material ad vantages are gained among which is the entire elimination of the usual time margin.

Other features including novel circuit details will be more apparent as the description progresses.

The drawings when placed in the form of a rectangle with Fig. 1 in the upper left hand corner, Fig. 2 in the lower left-hand corner, Fig. 3 in the upperri ht-hand corner, and Fig. 4 in the lower rig t-hand corner, diagrammatically illustrate a means for carrying out the present invention. Fig. 1 includes a group of subscribers lines with the means for establishing connections to the central ofiice trunks. Fig. 2 includes the means for makin incoming and outgoing connections to t e link circuits. Fi 3 is a representation of a central ofiice trun circuit. Fig. 4 is a representation of a link circuit.

A station to station call.

In the drawings, three station lines are shown which, in the system, would be sta tions 1, 2 and 9, here numbered 101, 102 and 109 respectively. Individual to each station line, there is a line relay L, whose function is to respond to outgoing calls from the substation and to initiate circuit movement for connecting the calling line to the first idle link circuit. Also, individual to each line is a cutoff relay 00 whose function is to respond to the seizure of a line on an incoming call to cut off the line relay and prevent the taking of an idle link circuit, as on an outgoing call. This role also responds to the seizure of an idle lin circuit on an outgoin call to cut off the line rela and remove t he calling condition set up y said line relay. Individual to each link circuit there are a number of relays whose functions are as follows:

Relay F operates to join a calling line to a. link circuit, 7

Relay G operates to connect a link circuit to a called line,

Relay TD operates to connect the common numerical leads from the counting relays to the G rela s of the link circuit whereby a particular relay is operated,

Relay P is an impulse-responsive relay and functions in the manner of the wellknown line relay, respondin to impulses propagated and transmitted rom the substation, and sup lies. through its windings, talking battery For the calling substation.

Relay H is a holding relay and functions as the first slow rela in the modern stepby-step systems. This relay remains energized during the sending of impulses, although its circuit is continually opened for short intervals during this time.

Relay FT is an allotter relay. lVhen the link circuit is idle, this relay is in normal osition and in this condition renders the ink circuit selectable. When the link circuit is seized, this relay becomes energized and transfers the selectable condition to the next idle link circuit. It also functions to put relay H under direct control of relay P.

Relay TK is a trunking relay. When a trunk, rather than a local connection, is desired, this relay operates in the same manner as the relay G and acts to cause a direct connection between a calling line and an idle trunk and to then restore the link circuit to normal.

Rela ST is a stepping relay controlled by relay to transmit the dial impulses to the counting relays.

Relay ON is a test relay and depending on the busy or idle condition of the counting relays will either remain deenergized or become energized, thereby causing the operation of a relay A associated with the counting relays to remove the condition which marks the counting relays as idle. When the counting relays are in use, relay ON fails to operate thereby preventing relay ST from locking and performing its proper functions.

Rela TA is a slow-to-release relay effective y responsive to the train of impulses transmitted from the substation. A the end of the last impulse, relay TA deenergizes and extends a ground throu h the contacts of relay TC to energize relay D which. in turn; causes the energization of a particular rela G.

RelayT is a fast relay indirectly controlled by relay TA. Its function is to connect ground from the contacts of relay TA after it has been deenergized to the winding of relay TD. It also acts to transfer a locking .ound from the back contact of a relay associated with the counting relays to the back contact of a rela C also associated with the common relays. The function of relays A, B and C will be bro ht out hereinafter.

ay TB is directly responsive to relay TA and is of the slow-to-release type whereb after the relay TA is deenergized, relay 'I B still maintains its armatures in its attracted position for an interval long enough to cause the proper energization of the relay TD.

Relay ET is a busy test relay. It operates when the called line is busy, cuts olf the sleeve of the called line from relay TR and transmits busy tone to the calling line. It opens the circuit of relay ST to prevent any dlsplacement of the apparatus, should the calling subscriber attempt to dial again. Relay BT also acts as the battery feed supply coils for the called line.

Relay TR is a relay responsive to the idle condition of the called line. When BT fails to operate TR becomes energized and opens the circuit of relay ST, in this way performing the same function as that pointed out for relay BT. Upon operation it marks the called line as busy.

Relay GO starts the ringing interrupters and connects the called line to the ringing leads. This relay is-shortcircuited by the relay B when it n turn operates.

Relay R is the ringing relay. Upon the operation of relay GO, ringing current is supplied through one of its windings to the called line and, being a marginal rela will not attract its armatures until the su scriber, by taking his receiver oil the book, so reduces the resistance of the substation loop that sufficient current will flow to cause the energization of this relay. Thereupon, relay R short circuits the windings of relay GO and locks itself in a local circuit to maintain the conversational circuit closed.

Relays A, B and G which are common to the link circuits, have the following functions.

Relay A acts to supply dial tone to the link circuits to indicate the idle condition of the counting relays. When the subscriber removes his receiver from the hook and re lay F functions, if the counting relays are idle and he may therefore operate his dial sender, busy tone will be transmitted from a back contact of relay A to the calling subscriber. Relay A therefore also acts to cut off the dial tone and the operating battery from the link circuits when the countingarelays are in use.

elay B is a slow-to-release relay whose function is to preventthe locking of relays ST should a called subscriber fail to notice the absence of dial tone.

Rela C cooperates with relay ST. It cuts 0 the holding ground from relay ST so that upon the end of the im ulse, relay ST will be deenergized. Relay functions to operate the counting relays.

T e actions and interactions of these relays will be more apparent from a description of the establishment of a station to station call.

When the subscriber at substation 101 ro- "moves his receiver from its switchhook, line relay 201 becomes energized through a circuit including battery, winding of relay 201, inner armature and back contact of relay 203, the loop of substation 101, back contact and outer armature of relay 203 to ground. Relay 201 attracts its armatures and extends a circuit from battery, winding of relay 206, back contact and inner right-hand armature of relay 207, front contact and inner right hand armature of relay 208 (assuming the first link circuit to be in use), conductor 210, contact and armature 211 of relay 201, back contact and outer left-hand armature of re lay 203 to ground. Relay 206 is energized in this circuit.

Attention is here directed to the novel means for establishing control over the link circuit pointed out in the statement of the invention. Heretofore control was established over a link circuit by extending .a temporary connection transient in character to the release trunk and later replacing such connection by a permanent connection. Under such conditions if the transient connection were not maintained Ion enough the whole operation would fail. ill the present instance such an ob'ectionable time margin is entirely eliminate by making the first act of establlshing control over the link circuit, the energization of the holding relay 206. Note that here the usual operation is reversed; that is, the holding relay 206 is operated before the impulse relay 216. Another feature of the present invention is an out rowth of the above novel arrangement an consists in the control of the selecting circuit jointly b the holding rela and a third relay whic has heretofore liiyed no part in this connection. This third relay, the starterwire advance relay 207, functions to mark the link as bus by merely advancng the starter wire to t e next idle link and it is not until this rela has been energized that control over the se ecting circuit can be established.

At the same time that relay 206 is energized, a circuit is established from ground, outer left-hand armature and back contact of relay 203, armature 211 and its associated contact of relay 201, contact and armature 214 of relay 201, left-hand winding of relay 212, conductor 213, back contact and outer left-hand armature of relay 207, front contact and outer left-hand armature of relay 208 to battery. Relay 212 is energized in this circuit and through its three left-hand armatures, connects the calling line 101 to the link circuit 400. As soon as relay 212 is energized a ground is extended from the right-hand armature 'of relay 206, normal contacts of the inner left-hand armature of relay 215, outer left-hand armature and; front contact of relay 212 winding of cut off relay 203 to battery. Relay 203 becomes ener 'zed and cuts ofi' the line relay 201 whic in turn becomes deenergized. The loop of the callin substation now causes the energization o relay 216 whereupon a circuit is extended from ground, winding of relay 401, conductor 214, left-hand armature and contact of relay 216, inner righthand armature and back contact of relay 215, left-hand winding of relay 207 to batterv. Bela 207 is ener 'zed in this circuit and throug its inner right-hand armature and its outer left-hand armature, extends the starting wires to the next idle link circuit in the same manner as relay 208 which through its ener ization extended the starting wires to the lll'lk circuit 400. The ground for operatin relay 207 is extended through the inner left-hand armature and front con tact of relay 207 to the windin of relay 206 whereby relay 206 is put un er the direct control of relay 216. Relays 206 and 207 being now energized, a circuit is extended from mid, the outer right-hand armature and ut contact of relay 207, left-hand armature and front contact of relay 206 to the right-hand winding of relay 207 so that re lay 207 remains energized as long as the holding rela 206 is energized. Through the outer rig t-hand armature of relay 207 and the left-hand armature of relay 206, ground is placed on conductor 402 thence through the outer left-hand armature and back contact of relay 403, the back contact and inner left-hand armature of relay 404, to the left-hand windin of relay 405. The left-hand'winding of rdlay 405, however, is short-circuited at this time due to a connection from ground, the ri ht-hand armature and front contact of re ay 216, conductor 406 to the winding of relay 405. The resistance 407 prevents the short-circuiting of the battery.

Since the left-hand winding of relay 207 and the winding of relay 206 are supplied with current flowing through the windin V of relay 401, this latter relay is energize and, through its armature, causes the energization of relay 408 which, because its circuit is made through its back contact and armature, is alternately energized and deenergized so that it buzzes and sets up on the conductor 409, a tone. This tone is used as a dial tone and extends over the conductor 409, through the condenser 410, the outer left-hand armature and back contact of relay 411, conductor 412, back contact and outer right-hand armature of relay 419, the right-hand winding of relay 404 to battery. By the inductive relation of the two windings of rela 404 and their connection to the main con uctors of link circuit 400, this dial tone is transmitted to the calling substation 101. The connection is now in a stable condition awaiting the sending of a series of impulses from the calling substation.

Let it be assumed that the subscriber at substation 101 wishes to establish a connection to substation 102. He thereupon manipulates his dial to send a series of two impulses which are in the form of two short interruptions of his line circuit. Relay 216 responds to this by retracting its armatures twice in quick succession and finally moving them to the attracted position where they remain until the end of the conversation. Upon the first retraction of the armatures of relay 216, the ground is removed from the conductor 406 whereupon relay 405 becomes ener ized and attracts its armatures. Relay 405 %ocks up in a circuit including battery, the right-hand winding of relay 405 front contact and inner right-hand armature of relay 405, normal contacts of the inner left-hand armature of relay 422, conductor 416 back contact and left-hand armature of relay 420. Immediately a circuit is established from battery, the right-hand armature and back contact of relay 411, conductor 414, right-hand winding of relay 419, normal contacts of the left-hand armature of relay 421, outer right-hand armature and front contact of relay 405 to the ground on conductor 402. Relay 419 becomes ener- 'zed, and through its left-hand winding, ront contact and inner left-hand armature of relay 419 locks to the ground on conductor 402. A circuit is now established from the ground on the left-hand windin of relay 405 through the front contact an outer right-hand armature of relay 405, outer left-hand armature and front contact of relay 419, winding of rela 421 to battery and ground. Relay 421 mes energized in this circuit and attracts its arma tures. Thereupon a circuit is established from ground, the alternate contact and right-hand armature of relay 421 to the wmdmg of relay 423. Relay 423 is energized in this circuit. Through its righthand armature it places a ound on conductor 418 thence to the winding of relay 411 to battery, whereby relay 411 is energized. The energization of relay 411 through the movement of its right-hand armature cuts oil the operating battery through the conductor 414 whereb relays O N of other link circuits may not energwed d1 unti l the counting relays have become 1 e.

By its left-hand armature, relay 423 places a ground on the windin of relay 422 which operates, whereby t e circuit previously described for the locking of relay 405' is now replaced by a circuit extendin from und, the left-hand armature am back contact of relay 424, conductor .417, must left-hand armature and alternate conmob of relay 422, inner right-hand armature and front contact of relay 405, and

right-hand winding of relay 405 to battery. A circuit is now established from battery, the winding of relay 424, conductor 415, front contact and inner right-hand armature of relay 419, left-hand armature and alternate contact of relay 421, outer right-hand armature and front contact of relay 405 to the ground supplied over conductor 402 for the left-hand winding of relay 405. This causes the energization of relay 424 whereby the lockin ground for the right-hand winding of re ay 405 is removed.

Through its right-hand armature, relay 424 places a ground on conductor 425 which extends through the normal contacts of the inner right-hand armature of relay 217 to the winding of relay 217. However, through the energization of relay 411 and the attraction of its inner left-hand armature, a ground is placed on the conductor 426 and thence to the other side of the winding of relay 217. Hence relay 217 is short-circuited at this time.

Relay 216 now attracts its armatures and through its right-hand ar mature and front contact again short-circuits the left-hand winding of relay 405 and since the ri hthand winding no longer has a hol ing ground, relay 405 retracts its armatures. Thereupon the ground for energizing relay 424 is removed and a circuit is established from battery, the winding of relay 424, the front contact and right-hand armature of relay 424, conductor 425, normal contact of the inner right-hand armature of relay 217, winding of relay 217, conductor 426, inner left-hand armature and front contact of relay 411, conductor 418, right-hand armature and front contact of relay 423 to ground. Relay 217 attracts its armatures and through its inner right-hand armature and alternate contact locks up to the battery connected to conductor 218. The movement of the armatures of relay 217 opens the circuit just described for relay 424 whereby it becomes deenergized. The retraction of its armatures by relay 405, opens a circuit for relay 421 but since this is a slow-to-release relay, it does not retract its armatures until after the whole series of impulses has been sent. When relay 216 again retracts its armatures on the second interruption of the line circuit, by removin the short-circuit from around the left-han winding of relay 405, relay 216 again causes the energization of relay 405. Again relay 424 is energized and, in a similar manner, establishes a short-circuit around the winding of relay 219. When relay 216 attracts its armatures at the end of the series of impulses, relay 405 is shortcircuited as before and the ground is again removed from conductor 415 by the movement of the outer right-hand armature of uently opened whereby after a relay 405. Counting rela 219 now becomes energized in series wit relay 424, and through the movement of its armatures deenerglzes relay 424 at the same time locking up to battery and conductor 218.

It will thus be seen that two interruptions representing th digit 2 will cause the energization of the first two counting relays 217 and 219, so that the ground will be extended from the left-hand armature and back contact of the last countin relay 221, the lefthand armature and bacl contact of the next counting relay 220 and so on until it reaches the left armature and front contact of the last counting relay to be operated, that is, relay 219, where the ground will be extended through the front contact to conductor 222. In a short interval of time, relay 223 will be energized whereupon conductor 222 will be extended throu h the armature 224 and front contact of re ay 223 to the windin of relay 225.

Tlpon the retraction of its armatures by relay 405, the circuit for relay 421 is permashort interval relay 421 becomes deenergized. A circuit is then established from round, the normal contacts of the right-hand armature of relay 421, the outer left-hand armature and front contact of relay 422, conductor 427, winding of relay 223 to battery, whereby relay 223 is energized with the resultant energization of relay 225 as just described.

Let it be assumed that substation 102 is at this time busy. A ground on the third wire will then be connected through the outer left-hand armature of relay 225 to conductor 428, the right-hand armature and alternate contact of relay 422, the outer right-hand armature and front contact of relay 419, the right-hand winding of relay 404 to battery. Relay 404 becomes energized in this circuit. A short interval afterrelay 421 has become deenergized, relay 423 retracts its armatures resulting in the deenergization of relay 422 and the deenergization of relay 411 whereby relay 223 and the counting relays are restored to normal, relay 225 having locked up through its right-hand armature and contact to ground at the right-hand armature of relay 206.

A source of busy tone 429 is now connected through the back contact and outer righthand armature of relay 403, the front contact and middle left-hand armature of relay 404, the lower normal contacts of the righthand armature of relay 422, the front contact and outer right-hand armature of relay 419 to the windlng of relay 404, whereby through the inductive relation of the two windings of relay 404, busy tone is transmitted to the calling subscriberfs station.

The calling subscriber now replaces his receiver on its hook whereby relay 216 becomes deenergized. Since this is more than a short interruption, the circuit including the left-hand armature and front contact of relay 216, the inner right-hand armature and back contact of relay 215, the inner lefthand armature and front contact of relay 207, the winding of rela 206, is broken so that relay 206 becomes eenergized. Thus, the round is removed from the right-hand win ing of relay 207 and it becomes deenergized. The ground being removed from conductor 402, relay 419 becomes deenergized and relay 206 by opening its contact at its right-hand armature, removes the ground for holding relays 225, 212 and 203 whereupon the connectlon is entirely restored to normal.

Let it be assumed that substation 102 is idle. Therefore, as soon as relay 421 becomes deenergized the circuit for the energization of relay 223 which connects the grounded lead 222 to relay 225, is established as before. A circuit will now be established through battery, the winding of relay 204, the outer left-hand armature and front contact of relay 225, conductor 428, right-hand armature and alternate contact of relay 422, outer right-hand armature and front contact of relay 419, right-hand winding of relay 404 to battery, but neither relay 204 nor 404 become energized in this circuit. In a short interval, relay 423 becomes deenergized and in turn relay 422 becomes deenergized. Thereupon this circuit is changed to lead now through the upper normal contact of the right-hand armature of relay 422, the outer le -hand armature and back contact of relay 404, the left-hand winding of relay 403 to ound. This causes the energization of dth relays 204 and 403 whereupon relay 204 is energized directly from the ground on conductor 402 through the outer left-hand armature and front contact of relay 403 to conductor 428, relay 403 in the meantime having locked up through its right-hand winding, inner armature and front contact to conductor 429, the normal contact of the inner left-hand armature of relay 215, front' 7 contact and ri ht-hand armature of rela 206 to ground. he ground on the rightand armature of relay 403 is also extended through the winding of relay 430, the lefthand armature and back contact of relay 431 to battery. Relay 430 becomes energized and through its inner left-hand armature and front contact places ground on conduc- -tor 432 which extends to the winding of relay 433, and thence to battery whereby relay 433 becomes energized and starts the ringing relays as will be pointed out here inafter. This places ringing current on conductors 434 and 435. A ringing circuit is now established from ground, conductor 435, front contact and outer left-hand armature of relay 430, front contact and inner left-hand armature of relay 403, through winding 437 in the following manner:

the loop of the called substation, front contact and right-hand armature of relay 430, right-hand winding of relay 431, conductor 434 to the other terminal of the source of ringing current. Ringin current flowing in this circuit is insu cient to cause the energization of relay 431 until the called subscriber removes its receiver from its hook. At that time, rela 431 becomes energized and by moving its left-hand armature, short circuits the winding of relay 430 which becomes deenergized. Relay 431 establishes a locking circuit through the left-hand winding of relay 431, left-hand armature and front contact of relay 431' to the ground on conductor 429. The conversational circuit is now established.

Talking battery is supplied to the callin line through the windings of relay 216 an to the called line thru the windings of relay 404. It is immaterial at this time whether relay 404 becomes energized or not as the movement of its armatures is without effect.

At the end of the conversation when the calling subscriber replaces his receiver on its hook, relay 216 becomes deenergized and the connection is restored to normal in the manner described.

Busy tone and ringing circuits.

The operation of the busy tone and ringcircuits will now be described. When re a 401 becomes energized as before descri ed, relay 408 is caused to buzz, placing on conductor 409 a tone which may be transmitted steadily through condenser 410 as a dial tone to the calling subscribers and which may also be transmitted through condenser 436 to the primary winding 437 of the busy tone trans ormer and thence through the back contact and left-hand armature of relay 438 to a ground on conductor 441 which may be supplied either at the contact and armature o relay 439 or at the contact and lefthand armature of relay 433. Relays 438 and 440 are operated by this ground to alternate- 1y open and close the circuit of the primary When ground is placed on conductor 441, it is extended through the right-hand armature and back contact of relay 438 thru the winding of relay 440, thence through the resistance 442 to battery. Relay 440 becomes energized 1n this circuit and connects the ground on conductor 441 through the left-hand armature and front contact of relay 440 thru the winding of relay 438, thence through the re- 1 sistance 443 to battery. The energization of relay 438 opens the circuit for relay 440 hereby after a short interval it becomes de energized. The retraction of its armatures by relay 440 in turn opens the circuit for relay 438 whereby after a short interval it becon'es deenergized. Thereupon the circuit for the energization of relay 440 is again energized,

established and so on. It will thus be seen that relays 438 and 440 are alternately enerized and deenergized and being sloW-to-reease relays, the resultant action is a slow making and breaking of the circuit including the primary winding of transformer 437. Thus the source of busy tone 429 is made periodically efi'ective.

The ound on conductor 441, placed there by the eft-hand armature of relay 433, may be periodically extended through the wrighthand armature and back contact of relay 440, the outer right-hand armature and front contact of relay 433 to conductor 444. From thence it is extended through the righthand armature and back contact of relay 445, the winding of relay 446, the back contact and inner left hand armature of relay 445 to batter Relay 446 is energized in this circuit. XVhen relay 440 becomes energized, the ground is removed from conductor 444 whereupon a circuit is established from battery, the inner left hand armature and back contact of relay 445, the windin of relay 446, front contact and right-ban armature of relay 446, the left-hand winding of relay 445 to ground. The current in this circuit causes the energization of relay 445, whereupon the right hand winding of relay 445 is now included in the circuit just traced. When relay 440 again becomes deground from conductor 441 is connected to conductor 444 and thence through the front contact and right-hand armature of relay 445, the right-hand winding of relay 445 to battery. The ground connected at circuiting of the left-hand winding of relay 445 and the winding of relay 446. Relay 446 becomes deenergized. In a' short interval relay 440 again becomes energized. Thereupon the ground is removed from con ductor 444 which results in the deenergization of relay 445. It will thus be seen that the pair of relays 445 and 446 cooperate to close the left-hand armature and front contact of relay 446, once for every two movements of the armature of relay 440. In other words, the movement of the pair of relays 445 and 446 is half as fast as the movement of the pair of relays 438 and 440. The pair of relays 445 and 446 18 used to periodically place battery through the outer left-hand armature and back contact of relay 445, the lefthand armature and front contact of relay 446, through conduotor 447 so that current will flow through either the right-hand or left-hand primary windings of the source of ringing current 448 to the outer left-hand armature and contacts of relay 449, the front contact and inner left-hand armature of relay 433 to ground. This battery connection to conductor 447 is thus made only when relay 446 is energized and relay 445 is deenergized this point causes the shortand since this condition obtains only once during every other closure of the right hand armature and back contact of relay 440, the ringing period will be one third as long as the silent period. In other words, the pair of relays 438 and 440 constitute a primary interrupter arrangement delivering impulses in cycles in which the circuit closures and the circuit breaks are uniform in duration and this device controls a secondary interrupter device, comprising relays 445 and 446, which delivers impulses in cycles in which the circuit closures and the circuit breaks are non-uniform in duration and of a lower frequency than the impulses delivered by the said primary interrupter deyice. The pair of relays 449 and 450 act in exactly the same manner as the pair of relays 438 and 440. Both being fast relays, their vibration is fast enough to cause the currents flowing in the prima y windings of the transformer 448 to set up an alternating current of the proper frequency for ringing purposes in the secondary windings of the transformer 448.

Thus, when a relay such as 430 becomes energized, in turn causing-the rela 433 to become energized, the ringing re ays are started and a source of placed on conductors 434 and 445. When :1. called line has been found busy, a circult is established from ground, windin of relay 401, winding-of relay 439, secon a of transformer 429, conductor 451, ba ii contact and outer right-hand armature of relay 403, front contact and middle lefthand armature of relay 404, lower normal contacts of relay 422, front contact and outer right-hand armature of rela 419, ri hthand winding of relay 404 to li attery. his causes the energization of relay 401 and 439 whereby a source of busy tone 429 is made effective and throu h the inductive relationv of the windings o relay 404 busy tone is transmitted to the calling subscriber.

Central oflt'ce trunk equipment.

Each central oflice trunk comprises a number of relays. keys and lamps whose funct ons are as follows: Associated with each trunk are a number of relays TG, one individual to each calling line'by means of which the trunk may be directly connected to a calling line.

Relay FF is-a trunk relay similar in function to the relay TD of the link circuit here tofore described. When relay FF iso erated, it connects the leads from the re ays F to the windings of the relays TG whereby a corresponding one of the relays TG is operated.

Relay KR is a relay somewhat similar in function to the relay FF. This relay is operated when any one of the selecting keys is operated and connects through its ringing current is armatures the numerical leads from the selecting keys to the windings of relays TG whereby a particular one of the relays TG mfg be 0 rated.

elay T is an advance relay similar in function to the relay FT of the link circuit heretofore described. Its function is to transfer the selectable condition of the trunk from one which has just been seized either for an outgoin or an incoming call tothe next trunk. hen the trunk is idle, relay FT is in its normal osition, but throughout the use of the trunk, FT is energized and transfers the lead 103 from the first in-order trunk to the next in-order trunk.

Relay PH is a holding relay similar to the combined functions of relays P and H of the link circuit. This relay remains energized throughout the conversation and supplies a holding ground to which other relays in the trunk circuit may be locked. When this relay becomes deenergized at the end of the conversation, it removes this holding ground whereby all energized rela s are restored to their normal condition. elay PH functions as a holdin relay on outgoing calls until relay C comes up after which the two co-operate.

Relay P is an impulse relay which supplies through its windings, current to the calling substation until the central otlice operator has responded. It also functions to put a calling condition on the central ofiice end of the trunk .to call in the operator.

Relay CH is a slow releasing relay whose function is similar to that of relay PH. Relay PH functions as a holding relay on an outgoing call and CH on an incoming call, though both cooperate during the-:onversation. This relay responds in effect to the handling of the trunk at the central ofiice end, and although being slow to release, will not follow interruptions which will cause the relay S to vibrate its armatures. Relay CH is operated whenever the supervisory relay S operates or the line relay L operates.

Relay GS functions to close the connec tion between calling and the called ends of the trunk circuit 110 after the central ollicc operator has answered or in general when conditions have been fulfilled to render conversation over the trunk possible. Relay GS substitutes for relay P in holding relay H energized.

Relay G0 is similar in function to relay GO of the link circuit. It operates to start the ringing interrupters and to connect ringing current to the circuit on a call from the central office to a station in the PBX.

Relay R is a ringing relay through whose right-hand winding, ringing current is suphed. When the called station answers, reay R is energized and through its left-hand contacts short-circuits the winding of relay GO whereupon the ringing current is cutoff from the called line and relay R locks up to maintainthe winding of relay GO whereby there is no possibility of placin ringing current on the called line after suc line has answered.

Relay S is a supervisory relay in series with a complete central office connection, and as such, responds to the switchhook of the subscriber within the PBX.

Relay L is a line relay responsive to an incoming call from central ofiice to the PBX. This relay is also responsive to the answer of the central oliice operator upon an outgoing call to the central otiice whereby the relay GS is o erated to cut-ofl' the impulse relay P and c ose the conversational circuit.

Relay SS is a heavy duty relay controlled by the supervisory relay S since it has to carry a larger number of armatures than a supervisory relay could operate. It is directly responsive to the operation of the v supervisory relay.

-to the called line. When any station Relay TS is a transfer relay co-operating with the test relay to connect a windin of the test relay to the third wire of the ca led line on a connection from central ofiice to a station. If the line tests busy this relay maintains its armatures in an operated position and connects busy tone to the attendants telephone or, if she has released her listening key, to the central oilice.

Relay LT is a relay responsive to the actuation of the stations keys and functions to connect another winding of the test relay LT becomes energized.

he 18 operated relay While it is energized it connects one winding of the busy test relay to the called line and when it becomes deenergized it connects the otherwinding of the busy test relay to the called line. If the called line is bu busy test relay is energized while relay L is energized, and if the called line is idle busy test relay is not energized until after the relay LT becomes deenergized.

Relay ET is a busy test relay.

Relay SA is a relay res onsive to the actions of the supervisory re ay S. When the called party answers and the supervisory relay is energized, relay SA becomes ener- 'zed to cause the disconnection of the ard imp GL. If the subscriber at the BX substation manipulates his switchhook to call the attendant, then relay SA completes the circuit for the line lamp so that the line lamp and the buzzer may again be actuated.

Relay LK is a listening key relay responsive to the actuation of the listening key. It carries contacts which must be closed when the listening key is actuated, but which must not again be opened when the listening key is restored. It is, therefore a locking relay which looks up upon the actuation of the listening key and stays in this condition until certain other fimctions have been performed.

Relay IC is a line relay responsive to al- {ernating current for energizing the line amp. 7

Relay BH is a slow releasing relay which is affected by the manipulation of the switchhook at the PBX station, but due to its slow releasing features, prevents the dashing by the subscriber to reach the central oflice operator. Retardation coil here numbered 351 cooperates with the line relay to hold a bri ge across the central ofiice end for actuatin the supervisory lamp thereat. Retardation coil 368 functions to su ply talkingdbattery to the PBX when the isconnect an flashing key is being manipulated.

Relay D distinguishes between an outgoing and an incoming call. It operates immediately on an outgoing call so as to place the control of the circuit in the central oflice. On an incoming call it is operated on the response of the called subscriber and causes the operation of relay GS to cut the connection through to the called subscriber. It is not operated on an incoming call by the attendant subscriber and its non-operation at this time prevents the functioning of the relays which cut through the connection.

Lamp BL is a busy lamp and when illuminated indicates a busy condition of the trunk circuit to which it is individual.

Lam LL is a line lamp and when illuminated indicates an incoming call from the trunk circuit to which it is individuaL This lamp also functions as a recall lam as when an incoming call has been extend to a station and that station wishes to call in the attendant subscriber;

Lamp GL is a guard lamp to indicate to the attendant that she has established a connection through the manipulation of her keys which will prevent the restoration to normaghof the a paratus of the trunk circuit when e centra ofiice disconnects.

Key 354 is a dialing or splitting key by means of which the attendant may cut in on a busy trunk connection. This he opens the connection, holds the central 0 cc end to prevent disconnection thereat and connects her telephone set to the station in the PBX. This ke also renders efiective the station keys an the relay KB. 7

Key 365 is a'release key by means of which the attendant may disconnect the trunk from the called station if it is desired to transfer the call to another station.

Key 367 is a flash key b means of which the attendant may entire y disconnect the PBX trunk apparatus from the trunk or by means of w ich she ma simulate the action of a subscriber in quic y moving his receiver hook up and down to attract the attention of the central oflice operator.

Key 347 is a listening key by means of which the attendant may connect her telephone to the trunk circuit.

Key 36 1 is a no test key by means of which the attendant subscriber may make a connection to a line which is proved to be busy whereby she may apprise the subscriber that an important call is waiting. Keys 363 and 353 are two of a number of stations keys by means of which the attendant may set up a connection to any one of the stations within the PBX.

Station to central ofice connection.

\Vhen a subscriber, say at station 101, desires to establish a connection to the central ofiice, he removes his receiver from its switehhook and, in the manner heretofore described, seizes the first idle link circuit such as link circuit 400. To make a central otlice call, the subscriber dials a particular number indicated on the drawings as 0, whereby he causes a series of interruptions of his line circuit which results in the energization of counting relay 221. At the end of this series of impulses, relay 405 becomes deenergized and after a short interval, relay 421 also becomes deenergized. A circuit is thereupon established from ground, the normal contacts and right-hand armature of relay 421, outer left-hand armature and front contact of relay 422, conductor 427, winding of relay 223 to battery. Relay 223 attracts its armatures and a circuit is thereby established from ground, left-hand armature and front contact of relay 221, the common numerical lead 226, outermost right-hand armature of relay 223, winding of rela 215 to battery and ground. Relay 215 comes energized in this circuit and establishes a locking circuitfrom ground, winding of relay 401, conductor 214, left-hand armature and front contact of relay 216, inner right-hand armature and contact of relay 215, winding of relay 215 to battery and ground.

Relay 215,,through its outer right-hand armature establishes a circuit from ground, right-hand armature and front contact of relay 206, outer ,ri ht-hand armature and front contact of re ay 215, conductor 103, inner right-hand armature and front contact of rela 104, (assumin trunk 105 to be in use), 1nner right-ban armature and back contact of relay 106, winding of relay 107 to battery and ground. Ground on conductor; 103 is also extended through the back contact and outer ri ht-hand armature of relay 106 to the win ing of relay 112 which becomes energized. Relay 112, throu 11 its left-hand armature places a groufil on the win oi relay 106 but this has no efiect at time, since the round on c nd ctor 103 15 xtended through the back contact and left-hand armature of relay 106 to the other side of the winding of relay 106 whereby this relay is short-circuited until the ground is removed from conductor 103.

A circuit is now established from ground through the outer left-hand armature and front contact of relay 215, the outer righthand armature and front contact of relay 212, conductor 111, innermost left-hand armature and front contact of relay 107, winding of relay 108 to battery and ground. Relay 108 becomes energized in this circuit and establishes a direct connection from the line 101 to the trunk 110. Through its in ner left-hand armature and alternate contact, relay 215 placed a ground on the mnductor 429 to replace the ground supplied from the right-hand armature and contact of relay 206 through the normal contacts of the inner left-hand armature of relay 215 whereby the cut-01f relay 203 is maintained in an energized condition.

The energization of relay 112 besides placing a ground on the relay 106 also extends a ground over conductor 113 to the lamp 326 and thence to battery whereby the' lamp 326 is caused to glow, indicating that the trunk 110 h become busy. Ground on conductor 11 is also extended through the normal contacts of the outer left-hand armature of relay 316, the back contact and outer left-hand armature of relay 328 to conductor 115, thence through the inner left-hand armature and front contact of relay 108 to the windin of relay 203 to hold this relay ener -ze after relay 212 becomes deenergiz Upon the direct connection being established between line 101 and the trunk 110 through the agency of relay 108, relay 313 becomes energized. This circuit may be traced from battery, right-hand winding of relay 313, normal contacts of the inner right-hand armature of relay 314, back contact and inner right-hand armature of re lay 315, back contact and inner IeftPhand armatur of relay 316, middle left-hand armature and front contact of relay 103, through the loop of the substation 101, outer left-hand armature and front contact of relay 108, back contact and inner left-hand armature of relay 316, outer right-hand armature and back contact of relay 315, normal contacts of the outer right-hand armature of relay 314, left-hand winding of relay 313 to ground. Through its right-hand armature relay 313 extends a ground through the outer right-hand armature and back contact of relay317, conductor 318, contacts 319, the upper limb of trunk 110 leading to the central otlice, winding of line relay 321 at the central ofiice to batt ry he by relay 321 bec m s en rg ed and causes the line lamp 322 to glow. Through its left-hand armature, relay 313 extends a ground from conductor 214 to conductor 323 and thence through the normal contacts of the inner left-hand armature of relay 324, contacts 325, winding of relay 324 to battery. Relay 324 becomes energized and establishes a locking circuit from ground, front contact and right-hand armature of relay 112, conductor 116, inner left-hand armature and alternate contact of relay '7 324, contacts 325, winding of relay 324 to battery and ground.

Ground on conductor 113 is extended through the contacts 327 to conductor 114 thence through the right-hand armature and front contact of relay 108 the winding of relay 108 to battery and ground to lock relay 108 in an energized position.

Shortly after these relay movements have taken place relay 423 becomes deenergized, and immediately thereafter relay 422 also deenergizes but neither relay 403 nor 404 become energized since no one of the rela s G have been actuated. Consequently, nelther relays 430 nor 431 become energized and ringin current is not supplied over the outgoing eads of the link circuit as before. The deenergization of relay 423 causes the deenergization of relay 411 whereupon the holding ground for the counting relays is removed and they return to normal. The ground supplied to the winding of relay 215 1s disconnected by the deenergization of relay 221.

Shortly after the energization of relay 215, relay 206 becomes deenergized due to the openlng of its circuit at the back contact and inner ri ht-hand armature of relay 215. U on the eenergization of relay 206, the ho] ing circuit of relay 207 is broken at the front contact and left-hand armature of relay 206 and a ground is thereupon removed from conductor 402 whereupon relay 419 becomes deener ized. Due to the retraction of this rightand armature, relay 206 removes ground from the holding winding of relay 212 whereby it becomes deenergized. This, in turn, breaks the circuit of relay 216 which retracts its armatures and opens the holding circuit for relay 215 which now becomes deenergized. The retraction of its armatures by relay 206 removes the ground from conductor 103 whereby relay 106 becomes energized through the ground on the left-hand armature and contact of relay 112 but movement of the armatures of rela 106 does not cause the deenergization of re ay 112 due to a subleave the connection in this condition: line 101 is connected directly through the contacts of relay 108 to trunk 110, relay 313 is energized, holdin relay 112 energized and actuating a signa at the central oflice. The link circuit 400 has been entirely disconnected and is now in condition for use on another call.

The operator at the central oilice observing the glowing line lamp 322 inserts a plug (not shown) into the jack 330, whereupon a circuit is established from ground on the tip of the plug, the upper limb of the trunk 110, contacts 319, contacts 331, the two windings of relay 332 in series, the normal contact and outer left-hand armature of relay 333, the lower limb-of the trunk 110, contacts 334, ring of the cord to battery. Relay 332 is energized in this circuit. A circuit is now established from ground through the righthand armature and contact of relay 332, the winding of relay 335 to battery and ground. Through its alternate contact and righthand armature, relay 335 places a ground on the inner right-hand armature of relay 324, whence it is extended through contacts 336 to the winding of relay 314. Relay 314 is now energized and b means of its right-hand armatures cuts 0 relay 313 and extends the talking conductors of trunk 110 to the central ofiice, whereby line 101 is connected throu h to the operator. Upon the establishment 0% this connection the supervisory relay 337 is energized in series with the line. By means of its lowermost armature, relay 337 connects ground to the winding of relay This relay opens the circuit of relay 332 and by means of its inner left-hand armature maintains the ground on the winding of relay 335. Relay 333 also connects a shunt 338 across the winding of relay 337. This shunt is used to reduce the impedance of relay 337 so that talking current may flow through the connections easily. The connection is now in a stable condition and conversation may proceed.

At the end of the conversation, if the subscriber at the station 101 hangs up his receiver, relay 337 will become, deenergized causing in turn the deenergization of relay 333, but the circuit for relay 332 will again become effective and relays 335 and 314 will be maintained energized. When the operator removes her plug. relay 332 will become deenergized causing the deener ization of relays 335 and 314. Relay 313 aving decnergized, ground is removed from conduc tor 323 whereupon relay 112 becomes deener izcd after a short interval in turn causing t e deenergization of relays 106, 108 and If the operator at the central oflice should disconnect before the subscriber at the station 101 replaces his receiver, then relay 337 will become deenergized causing in turn the deenergization of relay 333. This causes the deenergization of relays 335 and 314 whereupon the circuit for relay 313 again becomes efiective and the connection is put in the condition it was prior to the answering of the operator at the central ofiice. If the subscriber at station 101 now hangs his re ceiver on its hook relay 313 becomes deener ized, and removes the round from con uctor 323 whereupon re ay 112 deenergizes. The latter removes ground from conductor 113 whereupon relays 106, 108 and 324 deenergize and the connection is restored to normal.

Central oflice to station call.

In this system, as in all small private branch exchange systems where the number of stations is so small that the regular employment of an operator is unwarranted, trunk circuit apparatus is located near one of the regular stations with means whereby this regular station may answer incoming calls; In the present case, it will be assumed that the ninth station here numbered 109, is that station which is assigned to answer such incoming calls from the central oflice.

When the operator at the central ofiice inserts a plug into the jack 330 individual to the trunk 110, a circuit is established from battery, the ring side of the trunk through contact 334, the outer left-hand armature and back contact of relay 333, the two windmgs of relay 332 in series, contacts 331, the upper limb of the trunk 110, contact 319 to the tip of the plug of the operators cord circuit and thence to ound. In this circuit rela 332 is energiz A circuit is now establlshed from ground armature and contact of relay 332, winding of relay 335 to battery and ground. Relay 335 marks the trunk 110 as busy in the following manner. Through its left-hand armature it places a ground on conductor 113. This and is extended through the lamp 326 to attery and ground, to cause the lam to glow and mark the trunk as busy at t e attendants station. This ground is also extended through contacts 327 to conductor 114 and through the normal conta'ctsof the outer;

left-hand armature of relay 316, the back contact and outer left-hand armature of relay 328 to conductor 115. Ground on couductor 113 is also extended through the winding of relay 106 to battery and ground whereby relay 106 attracts its armatures and changes the condition of this trunk so that an outgoing call from a subwation to a trunk will be switched onto the next idle The circuit is now in a stable condition and awaiting the next operation.

This consists in the placing of ringing current on the trunk by the operator at the central oflice. Ringing current now flows from the ring of the operators plug into the lower limb of the trunk 110 through contact 334, the left-hand winding of relay 339, condenser 340, theback contact and outer left-hand armature of relay 314, contacts 331, contacts 319, the upper limb of the trunk 110 to the tip of the operators cord and thence to ground. This causes the energization of relay 339 which locks up over a circuit from battery, righthand winding of relay 339, the front contact and right-hand armature of relay 339, the back contact and outer left-hand armature of relay 317, the normal contacts of the outer left-hand armature of relay 341, to conductor 113. which is grounded through the contact and left-hand armature of relay 335. This ground is also extended through the right-hand armature and upper front contact of relay 339 to the line lamp 342 and thence through the buzzer relay 343 to battery and ground. Relay 343 attracts its armature and establishes a circuit from ground, the armature and front contact of relay 343, contacts oikey 344, right-hand winding of impedance coil 345, back contact and armature of buzzer 346, winding of buzzer 346, and left-hand winding of impedance coil 340 to battery and ground. As long as this circuit is closed at the left-hand armature of relay 343, the buzzer 346 w ll operate and the noise produced thereby Wlll attract the attention of the attendant subscriber. It is only necessary that the operator at the central ofiice place ringing current on the trunk 110 for a short interval, for relay 339 locks into its operating position and remains there until the attendant subscriber answers the telephone.

Let it be assumed that the subscriber at station 109 attracted by the noise of the buzzer 346 operates the key 347. A circuit will then be established from ground, contact 348, contact 349, back contact and lefthand armature of relay 350, guard lamp 351 to battery and ground. causing the lamp 351 to glow. This ground is also extended to the winding of relay 317 which attracts its armatures and unlocks relay 339 whereby line lamp 342 is extinguished. Through the main contacts of-key 347 the circuit 352 is connected to the normal contacts of the right-hand armatures of relay 356 and thence to the upper and lower limbs of trunk 110 leading to the central ofiice. After manipulating the key 347, the attendant-subscriber at station 109 removes the rece1ver from its hook, and is directly connected to the trunk toward the central oflice. Cu rrent is now supplied from the cord at the central office over the trunk 110, the circuit 352 to the siwcribers station 109, including supervisory relay 337 in series. This relay attracts its armature and establishes a C11- cuit from ground, the lowermost armature and front contact of relay 337, the winding of relay to battery and ground. Relay 333 attracts its armatures and by means of its inner left-hand armatures grounds the winding of relay 335. Through its outer Ieft-ha nd armature, relay 333 closes the shunt 338 about the Winding of relay 337. By means of its right-hand armature, relay 333 opens the shunt around the middle winding of retardation coil 351 so that now there is a shunt circuit across the trunk from the upper limb through contacts 331, lefthand winding of relay 332, the three windings in series of retardation coil 351, the right-hand armature and front contact of relay 317 to the lower limb of the trunk. The connection is now in a stable condition awaiting the next operation.

Let it be supposed that the calling party wishes to communicate with the subscriber at station 102. The attendant subscriber, upon being so informed, will manipulate her dialing key 354 and the station key 353 individual to the station 102. The dialing key 354 will close a ground through its upper contact to the winding of relay 356 and thence to battery and ground whereupon the relay 356 becomes energized, and attracts its armatures. Through the righthand armatures, the attendant subscribers station 109 is switched from the central office end of the trunk to the subscribers end of the trunk and talking battery is sup lied to the attendant subscribers telep one through the windings of relay 313, the normal contacts of the right-hand armatures of relay 314, the alternate contacts of the right-hand armatures of relay 356, the main contacts on the key 347 to the attendant subscrihers station 109. Relay 313 attracts its armature and connects the ground on conductor 214 through its left-hand armature and contact to the conductor 323 and thence through the front contact and outer right-hand armature of relay 106 to the winding of relay 112. Relay 112 is energized in this circuit and places a ground, through its right-hand armature, on conductor 116.

As the relay 356 switched the loop of the attendant subscribers telephone from the trunk, relay 337 becomes deenergized allowing re'ay in turn to become deenergized whereupon a circuit is established from the upper limb of the trunk through contacts 331, the left-hand low resistance winding of the line relay 332, the right-hand low resistance winding of retardation coil 351, right-hand armature and contact of relay 333, left-hand low resistance windin of retardation coil 351, and the inner righthand armature and front contact of rela. 317 to the lower limb of the trunk. This connection acts as a bridge to maintain the supervise relay at the central ofiice energized whi e the attendant subscriber is not olding the trunk.

The mani ulation of key 353 results in a connection rom ground, fight-hand contact of key 353, left-hand armature and front contact of relay 356, the outer left-hand armature and back contact of relay 355, the winding of relay 120, the outer left-hand armature and back contact of relay 324 to battery and ground. Relay 120 attracts its armatures and connects the numerical leads from the station keys to the windings of the various relays TG. Thereupon a circuit is established from ground, the lefthand contact of key 353 armature 121 of relay 120, winding of relay 122 to battery and ground. Relay 122 attracts its armatures and locks itself to the ground on conductor 114, at the same time connecting station 102 to the trunk 110. A circuit is at the same time established from ground, the right-hand contact of key 353, the lefthand armature and front contact of rela 356, the outer left-hand armature and bac contact of relay 355, the left-hand winding of relay 328 and the outer left-hand armature and back contact of relay 324 to battery and ground. Relay 328 is energized in this circuit. A circuit is now established from ound on conductor 113, contacts 358,

ront contact and inner left-hand armature of relay 328, winding of relay 316, contact 357, outer right-hand armature and front contact of relay 328, outer leftrhand armature and back contact of relay 324 to battery and ground whereby relay 316 becomes energized and locks itself to conductor 313 through its outer right-hand armature and front contact independent of the inner lefthand armature and front contact of relay 328.

A circuit is now established from battery, right-hand winding of rela 355, normal contacts of the inner rightand armature of relay 355, front contact and outer lefthand armature of relay 328, to conductor 115. If the called line is busy then this conductor will be grounded and relay 355 will be energized. Upon the movement of its armatures, it looks itself into a circuit including its right-hand winding, the alternate contacts of its inner right-hand armature, contacts 359, contacts 360, conductor 113 and thence to ground at the inner lefthand armature of relay 355. A circuit is thereupon established from battery, the back contact and outer left-hand armature of relay 324, the right-hand winding of relay 328, front contact and inner right-hand armature of relay 355 to ground on the conductor 113 whereby relay 328 is maintained energized. A circuit is now established from ground, the winding of relay 401, the

winding of relay 439, the secondary winding of busy tone transformer 429, conductor 451, the outer right-hand armature and front contact of relay 355, the front contact and inner lefthand armature of relay 316, the inner right-hand armature and back contact of relay 315, the normal contacts of the inner right-hand armature of relay 314, and right-hand winding of relay 313 to battery and ground. This causes the energization of relay 439 and relay 401 whereby busy tone is impressed on the transformer 429 and through the inductive relation of the winding of relay 313 such busy tone is transmitted to the attendantsubscribers station 109. The attendantsubscriber will thereupon release key 353 and restore key 354 to normal whereby she is a ain connected to the central ofiice end of t e trunk and informs the calling sub scriber that the station wanted is busy.

Ifthe called station 102 is idle then the circuit including the right-hand winding of relay 355 leads to battery and ground through the winding of relay 204 instead of to ground, and relay 355 does not become energlzed. Upon the releasing of key 353, the energizing circuit of relay 328 is broken and this relay restores its armatures to normal position whereupon a circuit is established from ground, left-hand winding of relay 355, the outer left-hand armature and alternate contact of relay 316, the back contact and outer left-hand armature of relay 328, conductor 115, inner left-hand armature and front contact of relay 122, the winding of relay 204 to battery and ground. Relay 355 is energized in this circuit and locks up through its right-hand winding over the circuits previous y described. A circuit is now established from battery, the back contact and outer left-hand armature of relay 324, back contact and left-hand armature of relay 361, winding of rela 315, outer righthand armature and balcli contact of relay 328, inner left-hand armature and front contact of relay 355 to ound on conductor 113. Relay 315 is energized in this circuit and relay'316 is short circuited whereby relay 315 attracts its armatures and relay 316 restores its armatures to normal. This results in the establishment of a circuit from ground, the left-hand armature and front contact of relay 315, conductor 432, winding of relay 433 to battery and ground. Relay 433 causes, in the manner hereinbefore described, the application of rin 'ng current to conductors 434 and 435. Finis ringing current may be traced from the source of ringing current, conductor 434, right-hand 361, front contact and inner ht-han armature of relay 315, inner 1:31am armature and back contact of relay 316, middle left-hand armature and winding of rela front contact of relay 122, through the 100 of the subscribers line 102, includin the ca 1 bell, the front contacts and outer eft hand armature of relay 122, back contact and middle left-hand armature of relay 316, outer right-hand armature and front contactrof relay 315, conductor 435 to ground whereby the bell at the substation 102 is intermittently actuated until the subscriber at that station answers. Ringing current is also conducted from the inner right-hand armature of relay 315, through the condenser 362, the normal contacts and the inner righthand armature of relay 314, the ri lit-hand winding of relay 313 to battery an round whereby, throu h the inductive action of the windings o relay 313, ringin tone is transmitted to the attendant-Sn scribers station 109 and she is thereby apprised of the fact that the bell at substation 102 is being rung.

Let it be assumed that the subscriber at the station 102 does not answer his telephone. If the attendant subscriber is still connected to the trunk through the actuation of listening key 347 she may apprise the arty at the central office end of the connectlon of this fact. If now the calling party abandons the attempt and hangs his receiver on its book, the operator at the central ofiice will remove the plug from the jack 330 and the attendant subscriber 109 will hang her receiver on its hook and restore the listening key 347 to its normal position. Thereupon relay 337 will be deenergized, and the source of current being removed from the tip and rin of the jac 330, line relay 332 will fail to me energized, whereupon relay 333 will become deenergized and relay 335, being new deprived of current, will restore its armature to normal. Ground is now removed from conductor 113 and all of the relays locked thereto will become deenergized and the connection restored to normal.

If, on the other hand, the calling lizarty at the central oflice end persists and ca for another subscriber, say at station 101, the attendant subscriber will operate key 365 which will release line 102 by removing ground from conductor 114 thereby ca the release of relay 122. Contacts 359 being opened, ground will be removed from the right-hand winding of relay 355 :and it will be released. Through the 0 mg of the inner left-hand armature of re ay 355 ground will be removed from the upper side of the winding of relay 315, whereu n it will re store its armatures to norma and ringing current will be switched off the trunk 110.

Through the opening of the contact at the left-hand armature of relay 315,the relay 433 will be restored.

The attendant subscliber now restores key 365 to normal and depresses her dialing key 354 and her station key 363, whereupon line 101 will be brought in on the connection in a manner similar to that described for line 102.

Let it be assumed that after the attendant subscriber has performed the operation necessary to cause the ringing of call bell at station 102, she restores her dialing key 354 and her listening key 347 to normal and hangs her receiver on its hook. Relay 317 is now restored to normal and with it lamp 351 ceases to glow due to the removal of ground at the contact 348 of the key 347 and due to the fact that relay 337 is not energized. If then station 102 does not answer and in time central office disconnects, relay 332 will become deenergized whereupon groundon the winding of relay 335 will be removed resulting in the removal of ound from conductor 133. This results In the extinguishing of the lamp 326, and the deenergization of relays 106, 112, 315, 328, 355 and 361, whereupon the connection is'restored to normal.

If now the station 102 is the proper station and the subscriber thereat answers in the regular way, upon the removal of his receiver from its hook, the current through the right-hand winding of relay 361 is increased sufficiently to cause the energization of this relay, whereupon a circuit is established from battery, the outer left-hand armature of relay 324, the left-hand Winding of relay 361, the armature and front contact of relay 361, the outer right-hand armature and back contact of relay 328, the inner left-hand armature and back contact of relay 355 to ground on conductor 113. Relay 361 is thereby locked in its energized position and relay 315 is shortcircuited since it now has ground applied to each side of its winding. Relay 315 thereupon restores its armatures to their normal position and a talking circuit is established be tween the attendant subscribers substation 109 and substation 102 with relay 313 supplying the talking battery. The attendant,

su scriber now restores key 354 to normal which causes the deenergization of relay 356. A circuit is now established from ground on conductor 214, the left-hand armature and front contact of relay 313-, the normal contacts of the inner left-hand armature of relay 324, contacts 325, the winding of relay 324 to battery and ground. Relay 324 attracts its armatures and locks itself to the ground on conductor 116 which is supplied at the right-hand armature of relay 112. The energization of relay 324 causes the establishment of a circuit from grounded battery, the winding of relay 314, contacts 336, the front contact and inner right-hand armature of relay 324, the righthand armature and alternate contact of relay 335 to ground, whereby relay 314 is energized and cuts off relay 313 to complete the circuit between substation 102 and central oliice end of the trunk 110. Talking battery is now supplied from the central office and therefore the supervisory relay at the central office and the supervisory relay 337 come under the direct control of substation 102. Relay 337 attracts its armatures and establishes a circuit from ground, the upper armature and contact of relay 337, the front contact and outer right hand armature of relay 324, the front contact and inner lefthand armature of relay 317, the left-hand winding of relay 341 to battery and ground. Relay 341 attracts its armatures and extends the ground from the outer right-hand armature of relay 324 through the inner left-hand armature of relay 341 to the winding of relay 350 whereupon this relay attracts its armatures and causes the guard lamp 351 to be extinguished. Relay 341 looks up through its right-hand winding, the front contact and inner rigl'it-hand armature, through contacts 366 to the ground on conductor 113. The energization of relay 324 by the removal of battery from its outer left-hand armatures causes the deenergization of relay 361. The connection is now in a stable condition and conversation may proceed. The attendant subscriber at station 109 may listen in on the conversation through keeping her key 347 actuated, but she may leave the conversation by restoring the key. In the latter case ground will be removed from contacts 349 originally supplied for the energization of relay 317, but relay 317 will not become deenergized at this time due to a substitute ground supplied from the upper armature and front contact of relay 337, the front contact and outer right-hand armature of relay 324, the alternate contact and outer left-hand armature of relay 341, the outer left-hand armature and front contact of relay 317, contacts 349 to the winding of relay 317,

Let it be assumed at this point that the subscriber at 102 has been called in error and therefore wishes to call the attendant subscriber back on to the connection. The natural thin to do in this case is to move the receiver ook at station 102 up and down several times in quick succession. Upon the first downward movement of the switchhook at substation 102, relay 337 becomes deenergized and removes the ground at its upper armature. This causes the deenergization of relay 317 whose locking circuit depending on this ground has just been described. Relay 350 is also deprived of current at this time but being slow to release does not restore its armature until after relay 337 is again energized. At this time ground is extended through the upper armature and front contact of relay 337, the front contact liill and outer right-hand armature of relay 321, the outer left-hand armature and alternate contact of relay 341, the outer lefthand armature and back contact of relay 317, the outer right-hand armature and front contact of relay 341, line lamp 342, the winding of relay, 343, to battery and ground. Upon each further deenergization of relay 337 in response to the movement of the receiver-hook at station 102, line lamp 342 will flash and when the subscriber at the station 102 ceases to move his receiver hook up and down line lam 342 will remain ermanently glowing. '1 e energization 0 relay 343 causes the buzzer 346 to be actuated and the attendant subscriber will be attracted by its buzzing. She will therefore come in on the connection by actuating listening key 347. Ground on contact 348 is now supplied to the winding of relay 317 which opens the circuit of line lamp 342 and locks to ground at the upper armature of relay 337. The subscriber at station 102 in conversation with the attendant subscriber informs such attendant subscriber that it is probably the subscriber at station 100 who is wanted, thereupon the subscriber at station 102 replaces the receiver on its hook and the attendant subscriber depresses the releasin keg 365, whereupon relay 355 is actuate an ground is removed as before from conductor 114 which causes the deenergization of relay 122. Relay 355 is deenergized through the opening of contact 359. The attendant subscriber now restores key 365 and actuates her dialing key 35% and station key 363 whereupon station 101 is called in a manner similar to that heretofore described.

Suppose the conversation is being held between substation 102 and .the central ofiice. At the end of the conversation when the subscriber at 102 hangs his receiver on its hook, supervisory relay 337 will become deenergized causl the deenergization of relay 333 and relay 31 but relay 332 now becomes energized in a circuit including its left-hand low resistance winding, the two low resistance windings of retardation coil 351, the ri ht-hand armature and front contact of relay 350 to the lower limb of the trunk 110. This condition holds for a moment only until 350 becomes deenergized whereupon the holding bridge to the central oflice is removed and the supervisory relay at the central oflice becomes deenergized. Relay 332 is, however, still ener through a circuit including its two win ings in series, the back contact and outer left-hand armature of re lay 383. The operator at the central 015% now removes her plug from jack 330 and relay 1332 becomes deenergized, whereu n und is removed from the winding 0 re- E; 335 which in deenergizing in turn moves the ground from the conductor 1i1 3 and all of the relays which are locked to this conductor are restored to normal thereby fully restoring the entire connection to normal.

Keys 117 and 118 are rovided for the use of the attendant subscri r station 109. If the station 101 calls for station 109 the call bell 120 will be actuated in the same manner as the call bell at any subscribers station and the attendant subscriber at station 109 responds by removing her receiver from its hook and actuating key 117. If, while hearing a conversation with a station in the PBX a central office call is established and the attendant subscribers attention is needed, she may actuate key 118 which will place a bridge 119 across her line circuit to hold the established connection so that she may then attend to the incoming call without causing an interconnection between the original connection and the newly established connection. On a connection between the central ofiice and a substation say 102, it is shown that substation 102 may call the attehgant subscriber by manipulating the receiver 00k at the station 102, thereby causing thelighting of line lamp 342 and the actuation of the buzzer 346. The manipulation of the receiver hook at station 102 is without efi'ect on the central ofiice since each time relay 337 becomes deenergized, a bridge comprising the low resistance winding of line relay 332 and the two low resistance windings of the retardation coil 351 are thrown across the line to prevent the supervisory relay at the central ofiice from actuating. In order to cause the actuation of the su rvisory relay at the-central oflice the attendfnt subscriber may manipulate her flashing key 367 which directly 0 us the trunk towards the central oflice. is also throws coil 368 on to the trunk 110 towards the subscribers station to supply talking battery in place of the central oflice battery.

What is claimed is:

1. In a tele hone system, tele hone lines, trunk lines, a ink circuit inclu ing two relays, means for seizing said link circuit, means for mani ula said link circuit to select one of sai telep one lines, means for controlling said relays in one uence to restore said link circuit to name means for manipulating said link circuit to select an idle one of said'trunk lines, and means responsive to said last means for controlling said relays in another sequence to restore said link eirc to normal.

2. In a tel hone stem, a link circuit, an impulse re ay an .a holding relay associated there'wi means for extendmg a connection to sai link circuit, means for energizing said holding rela prior to the energizatlon of said mp rela and means responsive to the extension 0 a connection to said link circuit for placing the control of said holding relay in said impulse relay.

3. In a telephone system, a link circuit, an impulse relay, a olding relay and a selecting circuit associated with said link circuit, said selectin circuit being under the joint control of sai impulse relay and said holding relay, means for extending a connection to said link circuit, means or energizing said holding relay prior to the enerizatlon of said impulse relay, means for renfiering said selecting circuit lnefi'ective at this stage, and means responsive to the extension of a connection to said link circuit for placing the control of said holding relay in said impulse relay and establishing control over said selecting circuit.

4. In a telephone system, a link circuit, an impulserelay, a holding relay, a starter wire advance relay and a selecting circuit associated therewith, said selecting circuit being under the joint control of said relays,

means for extending a connection to said link circuit, means for energizing said hold ing relay prior to the energization of said impulse rela and means responsive to the extension 0 a connection to said link circuit for placin the control of said holdin relay in sai impulse relay and establis ing control over said selecting circuit.

5. In a telephone system, a link circuit, an impulse relay, a holding relay, an advance relay and a selecting circuit associated therewith, said selecting circuit being under the joint control of said relays, a connecting relay fnr extending a connection to said link circuit, means for energizing said holding relay and said connecting relay, means responsive thereto forenerglizing said impulse relay and said advance re ay, and means responsive thereto for placing the control of the said holding relay in the said impulse relay and for establishing the said control over the said selecting circuit.

6. In a telephone system, a link circuit, an impulse relay, a holding relay, an advance relay and a selecting circuit associated therewith, said selectin circuit being under the joint control of sai relays, a connectin relay for extending a connection to said li circuit, means for simultaneously energizin said holding relay and said connecting re ay, means responsive to the energization of said holding relay for energizing said advance relay, means responsive to the energization of said connecting relay for energizing said impulse relay, and means responsive to the energization of said advance relay for shifting the control of said holding relay to said impulse relay and establishing the said control over said selectin circuit.

7 In a telephone system, telep one lines, link circuits, a trunk circuit, switching devices having permanently paired primary and secondary contacts for interconnecting said lines and said links and said trunk, means in said links under control of said lines for disconnecting a link from a line and causin the connection of said trunk to a line, sai means includin contacts of a switching device employed 1n connecting a said line to a said link.

In witness whereof, I hereunto subscribe my name this 12th day of July A. D.,'1920.

EDWARD E. HINRIcHsEN. 

